Clone REA279 recognizes the CD194 antigen, a seven-transmembrane G-protein–coupled receptor also known as C-C chemokine receptor type 4 (CCR4). CD194 is expressed on activated Tʜ2 cells, regulatory T cells, a subset of B cells, activated natural killler (NK) cells, basophils, monocytes, NK cells, and platelets. It is expressed at high levels by skin-infiltrating lymphocytes, at lower levels by lung and synovial fluid lymphocytes, but never by intestinal lymphocytes. CD194 is the specific receptor for the chemokines CCL17 (TARC) and CCL22 (MDC). Human peripheral blood regulatory T cells can be divided into two distinct populations based on the expression of CCR4. The majority of freshly isolated regulatory T cells express CD194 and represent memory-type regulatory T cells, while CD194 regulatory T cells require anti-CD3 antibody–mediated activation to acquire a regulatory activity. Depletion of 194+
T cells leads to Tʜ1-type polarization of CD4+
T cells and augmentation of CD8+
T cell responses to tumor antigens.